Wiedemann P, Kohen L (eds): Macular and Retinal Diseases.

Dev Ophthalmol. Basel, Karger, 1997, vol 29, pp. 1 7

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Recent Developments in Scanning Laser Ophthalmoscopy

Sebastian Wolf

Augenklinik der Medizinischen Fakultat der RWTH Aachen (Director: Prof. B. Kirchhof), Aachen, Germany

The scanning laser technique employs a new electro-optical principle that does not require optical image formation [1, 2]. This allows not only imaging of the retina with different wavelength [3] but permits a variety of new applications like angiography [4, 5], blood flow measurement [6, 7], reflectometry [8], retinal densitometry [9], microperimetry [10±12], and other functional testings [13±15]. Currently, several instruments for retinal imaging or functional testings using the scanning laser technique are commercial available from different companies.

Methods and Materials

The scanning laser technique has been described previously in detail [1, 2]. In short, for illumination a scanning laser ophthalmoscope uses a laser beam, focused by the optical system of the eye to a small moving spot, that is swept across the retina to form a rectangular raster. The light returning from the retina is converted by a high sensitivity solid state detector into an electronic signal from which a two dimensional image is constructed electronically [16]. From this electronic signal a standard video sig nal can be created and recorded with a video recorder. Furthermore, the electronic sig nal can be directly digitized and stored on a computer. Since in scanning laser systems only a small point of the retina is illuminated, only a very small area of the pupil is used for illumination and the rest is available for light collection. Therefore, scanning laser systems are highly light efficient and reduce light intensities for illumination of the retina for imaging by a factor of 100 1,000. This permits recording of a very high

Supported by DFG (Deutsche Forschungsgemeinschaft; Bonn; Germany) AZ Wo478/5 3.

number of images without reaching the maximum permissible light level for retinal ir radiance [17, 18].

Additionally, the optical principal of scanning laser systems allows different im aging modes. In the direct imaging mode the photodetector in the scanning laser sys tems accepts all light collected by the instrument. These nonconfocal images suffer from reduction of contrast due to light which is returned to the detector after being scattered or reflected by layers from outside the optical plane. In the confocal imaging mode a pinhole, which is conjugate to the laser focus, is placed in front of the detec tor [16]. The size of the pinhole determines the degree of confocality of the image, a small pinhole produces a highly confocal image. In the confocal mode the depth of field is very small producing optical sections of the fundus. Using an annular aperture instead of the small pinhole in front of the detector all light reflected from the focal plane is blocked and only stray light produces the image. This mode is called the in direct mode.

Another important feature of scanning laser systems is the ability to modulate the intensity of the illumination by varying the laser power by means of an acousto optic modulator. This allows to produce a graphic design on a patient's retina that is simulta neously viewed by the patient and observed on the fundus on the electronic image. Con trolling the acousto optic modulator by a computer a variety of psychophysical test can be performed.

Depending on the application different lasers are used as light source in scanning laser systems. For fluorescein angiography an Argon laser (wavelength 488 and 514 nm) is used. Additionally, autoflurescence images can be recorded with this laser [19]. For in docyanine green angiography and infrared imaging diode infrared lasers (wavelength: 788 and 820 nm) are used. Microperimetry is usually performed with HeNe laser illumi nation (wavelength 633 nm).

Results

Fluorescence Angiography

The distinctive features of scanning laser imaging improve the signal- to-noise ratio and thus enhance the contrast of the image as compared with standard imaging techniques. Beside the high quality of the fluorescence angiographic images (fig. 1, 2), the high frame rate (up to 60 Hz) allows a detailed analysis of the blood flow dynamics during angiography. The improved resolution of scanning laser angiograms made the acquisition of capillary flow velocities and the assessment of capillary density in the perifoveal network possible [7]. In various diseases and under varying physiological conditions capillary flow velocities and capillary density in the perifoveal network have been assessed [20±22]. These studies have shown that capillary density and flow velocity are reduced in patients with diabetes mellitus even without diabetic retinopathy [21]. Similar results were found in patients with systemic hypertension [22].

1

2

patient with occult CNV. Left: Fluorescein angiogram; right: ICG angiogram with visible CNV (arrows).

Autofluorescence Imaging

We used a standard confocal scanning laser ophthalmoscope (HRA, Heidelberg Engineering) for visualization and mapping of retinal autofluorescence. For excitation the argon laser blue line (wavelength 488 nm) was used at maximal intensity (300 lW/cm2). In the detection pathway an interference filter (band pass with > 90 % transmission for 505±700 nm) and a confocal stop were inserted. The images were digitally recorded. We

Fig. 3. Autofluorescence image (left) and fluorescein angiographic study (right) (30° field; HRA, Heidelberg Engineering) in a patient with geographic atro phy secondary to AMD. Note the decreased autofluorescence in the area of atro phy.

have analyzed the autofluorescence images of patients with age related macular degeneration (AMD). Areas of hyperpigmentation at the level of the RPE showed increased autofluorescence, whereas areas of depigmentation appeared hypofluorescent in the autofluorescent images in all cases (fig. 3). Patients with AMD demonstrated focal accumulation of fluorescent material ± most likely lipofuscin. Thus, the scanning laser technique combined with an image analyzing system may help to identify eyes at risk for the development of exudative AMD.

Microperimetry

Currently, only the scanning laser ophthalmoscope (SLO-101, Rodenstock Instr., Germany) can be used for fundus perimetry. For retinal imaging an infrared diodelaser (780 nm) is used. Background illumination and stimuli are generated with a HeNe laser (633 nm) modulated by an acous- to-optic modulator (AOM). The AOM is controlled by a microcomputer. In our system we use an Image Technology FG100-AT board. Our current software for fundus controlled perimetry allowed for static automatic microperimetry by means of a suprathreshold staircase strategy [10]. Light intensities could be varied between 0 and 27.9 dB above background. For clinical studies the first stimulus is presented with 10 dB, thereafter light intensities are increased by 4 dB after a correct answer and decreased by 2 dB, if the stimulus is not seen. During the test procedure a fixation cross

Fig. 4. Result of a microperimetry in a patient with an extrafoveal laser scar after laser photocoagluation for a well defined choroidal neovascularization. The numbers indicate the local retinal sensitivity expressed in dezibel.

(size: 36 × 36 min arc; contrast: 0 dB) is presented. Fixation and eye movements are controlled by manual fundus tracking. Therefore, it is possible to calculate the circle area that would encompass 75 % of all fixation points and the center of fixation. The radius of this circle is given to quantify fixation stability. Figure 4 shows the result of a microperimetry in a patient with AMD after laser photocoaculation for an extrafoveal CNV.

Discussion

The scanning laser ophthalmoscope provides excellent fluorescein and indocyanine green angiographic images [23, 24]. The high frame rate allows not only morphologic analysis of the angiograms but permits additionally the assessment of retinal hemodynamics [20±22]. Recently, visuali-

zation and mapping of fundus autofluorescence with the scanning laser ophthalmoscope have demonstrated the possibility to assess the metabolic activity of the RPE [19]. This technique may add a great deal of information about the correlation between accumulation of autofluorescent material in the RPE and the progression of AMD. Equally significant for the clinical assessment of macular diseases is the ability of the scanning laser technique to provide complex static or dynamic testing for identifying the retinal loci of functional deficits.

Additional clinical applications with scanning laser systems are currently under investigation. Further technical improvement will expand the clinical use of the scanning laser technique in the future.

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Priv. Doz. Dr. Ing., Dr. med. S. Wolf, Augenklinik der Medizinischen FakultaÈt der RWTH Aachen, Pauwelsstrasse 30, D 52057 Aachen (Germany)

Fax: (02 41) 8 88 84 08